Terrain texture limitation before going Black?

Hi there.
Im just starting out with unity, and am proceeding towards building my first game.

Upon adding lots of seemless textures within the Terrain painting tool, it seems i can only use the first 30 or so textures before they go black.
Is this a general limitation or is it a bug?

Ive tried looking throughout the site for answers, but no-one seems to ask this. Ih have seen someone amongst repies stating the same thing, but no-one answered.

Can anyone help?

a month and no reply :confused: